1. The Field of the Invention
The present invention relates to gingival retraction cords. More particularly, the present invention relates to chemically pre-impregnated silk retraction cords having high resistance to degradation over time.
2. The Related Technology
When performing various dental procedures, it is often desirable to retract the gingival tissue to prepare the patient's teeth for the procedure (e.g., to take an accurate and reliable impression of the tooth). Taking dental impressions, placing crowns, performing bridge work, or effecting other dental restorations are examples of procedures that benefit from retracted gingival tissue. A widely used method for retracting gingival tissue involves the use of retraction cords which are typically braided or knitted for increased strength, flexibility and resilience. Examples of gingival retraction cords may be found in U.S. Pat. No. 4,321,038 to Porteous, U.S. Pat. No. 4,522,593 to Fischer, U.S. Pat. No. 4,617,950 to Porteous et al., U.S. Pat. No. 4,892,482 to Lococo, U.S. Pat. No. 7,121,828 to Fischer et al., and U.S. Pub. No. 2008/0096164, which are incorporated herein by reference.
In addition to making it more difficult to take an accurate impression of the proper shape of a patient's tooth beneath the gingival margin, the gingiva (or “gums”) can also bleed if torn or damaged by a high speed cutting drill or burr used to remove tooth material preparatory to placing a crown. Bleeding may further interfere with taking a good impression because extravasated blood can prevent adequate cleaning and drying of the marginal area of the tooth prior to taking an impression and displace the impression material before it can set. Thus, the dual problems of contraction of the gingival cuff and the presence of hemorrhaging tissues make it impractical to simply take an impression following shaping of the tooth with a high speed drill or burr without retraction of the gingiva.
To control or inhibit gingival bleeding, retraction cords can be treated with a hemostatic agent. One type of hemostatic agent includes an astringent, which lock or seal off exposed blood vessels so as to arrest bleeding. U.S. Pat. Nos. 4,321,038, 4,522,593, 4,617,950 and 4,892,482, referred to above, discuss the use of astringents such as potassium aluminum sulfate, also known as “alum”. More powerful astringents include iron based salts, which are highly acidic and corrosive.
Highly acidic chemicals used to impregnate a retraction cord can adversely affect the strength and integrity of the retraction cord. Retraction cords are typically made of natural fibers, such as cotton, which are highly absorbent and can absorb and retain high quantities of a liquid astringent. However, applying a corrosive hemostatic agent to a retraction cord made of natural fibers such as cotton or other cellulose based fibers can quickly degrade the cord and reduce its strength and integrity, making it more likely to fray and/or fail during use. If fibers in the retraction cord fail during use, the dental packing instrument used to insert the retraction cord into the sulcus can slip through the fibers and potentially cut or injure the underlying tissue. In addition, fragments of the retraction cord can remain embedded between the tooth and gums, providing greater risk of infection. Frayed filaments can easily lodge within coagulum, which can be painful to the patient and result in a recurrence of bleeding when the cord is removed. Furthermore, the expandability and resilience of the retraction cord diminishes as the integrity of the knit or weave of the cord degrades.
In view of the foregoing, gingival retraction cords made from absorbent natural fibers such as cotton or other cellulose based fibers cannot be feasibly pre-impregnated with corrosive agents, such as iron based hemostatic agents, and then shipped in bulk to end users. Doing so would yield a cord that is so friable and weak as to be useless as a retraction cord. Instead, corrosive agents have been applied to natural fiber retraction cords by the dental practitioner chair side, just prior to use.
Man-made fibers, such as nylon and polyester, can be used to make gingival retraction cords that resist degradation by corrosive agents. However, cords made from man-made fibers are bulkier and far less absorbent than retraction cords made from natural fibers, such as cotton. As a result, such cords are less desirable than natural fiber cords.